1. Field of the Invention
This invention generally relates to an information transmission system. This invention particularly relates to a system for transmitting digital information while keeping it secret. In addition, this invention relates to a method of transmitting cipher information.
2. Description of the Related Art
In a conventional digital-information transmission system, digital data (digital information) to be transmitted is processed into a prescribed transmission format before being actually transmitted. Specifically, the digital data is divided into equal-size segments. A sync signal is added to the head of every data segment while a parity signal is added to the end thereof. The sync signal is designed for the identification of the first bit in the data segment. The parity signal is designed for the detection of an error or errors in the data segment. A set of the sync signal, the data segment, and the parity signal is a prescribed transmitted-signal unit block. In this way, the digital data to be transmitted is converted into a sequence of unit blocks inclusive of sync signals and parity signals. The sequence of unit blocks is transmitted.
During the transmission of a signal of the above-mentioned format, anyone can tap digital data in the transmitted signal if the signal format is disclosed.
For keeping transmitted digital data secret, it is usual to encrypt the digital data before the transmission thereof. The DES (Data Encryption Standard) is a typical method of encrypting digital information. According to the DES, digital information to be transmitted is encrypted in response to an encryption key on a block-by-block basis where every block has 64 bits. Similarly, a receiver side decrypts incoming information in response to a decryption key on a block-by-block basis. The encryption key in a sender side and the decryption key in the receiver side are the same. In the case where one bit in digital information encrypted by the DES changes to an error during the transmission, the 1-bit error causes a receiver side to conclude the whole of a block inclusive of the 1-bit error, that is, 64 bits inclusive of the 1-bit error, to be wrong since the block-by-block decryption is implemented therein. Thus, the DES tends to decrease the error correction capability.
Japanese book entitled “Modern Cipher”, written by Tatsuaki Okamoto and Hirosuke Yamamoto, published by Sangyo-Tosho Kabushikikaisha on Jun. 30, 1997, pages 73-75, discloses an output feedback (OFB) mode of the DES. The DES-OFB mode is usually employed for the transmission of digital information via a transmission line apt to cause bit errors. In the DES-OFB mode, encryption is used to generate keystream blocks. Specifically, an encryptor changes every input block into a keystream block according to the DES. The generated keystream block is fed back to the input side of the encryptor, and is used as a next input block. Thus, every keystream block is generated by encrypting the last one according to the DES. As a result, every keystream block corresponds to a pseudo random number. The generated keystream blocks are XORed with information blocks to get concealed information (cipher information) to be transmitted.
In such encryption-based information transmission systems, it is necessary that an encryption key is transmitted between an information sending side and an information receiving side, and is held in common by them. Generally, authentication or certification necessitating complicated calculations is used as a portion of a procedure of implementing the high-security transmission of an encryption key and enabling an information sending side and an information receiving side to hold the encryption key in common. Thus, the encryption-based information transmission systems require calculation devices of large circuit scales for performing the authentication or the certification. The calculations for the authentication or the certification take a relatively long time. For enhancing the security of transmitted information, it is usual to frequently and synchronously update encryption keys in an information sending side and an information receiving side. Generally, authentication or certification necessitating complicated calculations is used as a portion of a procedure of synchronously updating the encryption keys in the information sending side and the information receiving side.
In the DES-OFB mode, an initial value for an input block is changed to alter a scramble pattern or an encryption pattern in each of an information sending side and an information receiving side. For security, authentication or certification necessitating complicated calculations is generally used as a portion of a procedure of synchronously changing the initial values in the information sending side and the information receiving side.